1. Field of the Invention
The present invention relates to a method for exposure of an image captured by an image-capturing device.
2. Description of the Related Art
Image-capturing devices, for example, video cameras, digital still cameras, and cellular phones equipped with cameras, perform image processing on image data photoelectrically converted by image pickup elements, for example, charge coupled devices (CCDs) and complementary metal-oxide semiconductor (CMOS) sensors, and display images on display units, such as liquid crystal displays (LCDs) or store the processed image data on storage media, for example, flash memories and magnetic tapes.
In this process, the image-capturing devices perform automatic exposure control (hereinafter called AE control) to appropriately adjust exposure of images to be captured.
In AE control, an aperture that determines the amount of exposure of the image pickup elements, a shutter speed that determines the length of exposure time of the image pickup elements, and a gain in programmable gain amplifiers (PGAs) that amplify data read from the image pickup elements are controlled, and thus the luminance level of image data is appropriately controlled.
In AE control, the difference between a current luminance level of image data and a luminance level that is set as an appropriate value is calculated, and the aperture, the shutter speed, and the gain are controlled so that this difference is compensated for. However, especially in a case of moving images, when the luminance level of image data is rapidly compensated for, the viewability of the images is likely to be impaired in spite of the original intention. Thus, a technique has been proposed in conventional systems for causing the luminance level to smoothly converge at an appropriate value by setting a time constant on which a change in the luminance level depends. When the luminance level changes, the viewability of the images can be kept by setting the time constant. However, when a required change in the luminance level is large, the time required for the luminance level to converge at the appropriate value is long. Thus, a technique is discussed in, for example, Japanese Patent Laid-Open No. 2003-153073, in which a time constant is set so that the luminance level gradually converges at an appropriate level when the luminance of a subject changes, and the time constant is changed according to a required change in the luminance level. In this technique, when the required change in the luminance level is larger than a predetermined value, the time required to compensate for the luminance level can be shortened by decreasing the time constant.
However, even in the case where the time constant on which a change in the luminance level depends is set, when the amount of amplification of image data that is read from the image pickup elements by the PGAs is significantly changed, noise may occur in the image data depending on characteristics of the PGAs.
Moreover, when the aperture is significantly changed, noise occurs in the image data, as in the case where the amount of the gain is significantly changed, and substantial noise is produced when the aperture is changed, so that the noise is recorded when moving images are shot.